1. Technical Field
The present invention relates to a process for producing lithium titanate which is suitable for electrodes for lithium ion batteries used, for example, as backup power supplies for personal computers and for portable types of equipment and the like, and to lithium ion batteries using the same.
2. Background Art
The recent rapid development of technology in the field of electronics has allowed compact design and weight reduction in electronic equipment. Secondary batteries as driving or backup power supplies for this equipment are strongly required to be of compact design and light weight, and to have high energy density. Recently, development of larger electricity storage systems are also urgently required for electric automobiles and off-peak domestic electricity storage systems in order to reduce CO2 emissions. As new types of secondary batteries which can meet these requirements, secondary lithium batteries which have high energy density to volume have attracted attention.
Lithium titanate, presented by the formula, Li4Ti5O12 (termed Li4/3Ti5/3O4 below), is used as a material for secondary lithium batteries. As processes for production of these compounds, wet methods and dry methods are known (for example, unexamined published Japanese Patent Application, No. 309727/97, and Journal of Low Temperature Physics, Vol. 25, p. 145, 1976). Although wet methods can produce lithium titanate having good crystallinity, these methods require complicated processes, waste water treatment and the like, which poses problems of economic efficiency. To contrast, although the conventional dry methods are simple in process, lithium titanate by-products with formulas other than the above are produced. Furthermore, control of the atomic ratio of titanium and lithium (hereinafter referred to as the Li/Ti ratio) is difficult due to vaporization loss of elemental lithium and lithium compounds, and titanium dioxide as a raw material remains in the products. As a result, the method poses problem in that lithium titanate, Li4Ti5O12, cannot be efficiently produced.
The present invention was made to solve the above-mentioned problems. Objects of the invention follow.
(1) Providing a production process for efficiently producing lithium titanate using a dry method.
(2) Providing a process for producing lithium titanate, in which vaporization loss of lithium compounds in the sintering reaction can be suppressed, so that the Li/Ti ratio can be freely controlled in the range of 0.78 to 0.82 (i.e., around 0.80), and preferably in the range of 0.79 to 0.80.
(3) Providing a process for producing lithium titanate, in which persistence of titanium dioxide as a raw material can be suppressed.
According to research by the inventors, it was discovered that vaporization loss of lithium compounds can be extremely decreased, the Li/Ti ratio can be easily controlled, titanium dioxide as a raw material did not remain, and therefore lithium titanate compounds can be efficiently produced by employing a process in which specific sintering conditions are combined.
The invention was completed based on the above research. The invention provides a process for producing lithium titanate by presintering a mixture of titanium dioxide and at least one lithium compound selected from the group consisting of lithium carbonate, lithium hydroxide, lithium nitrate, and lithium oxide to prepare at least one of a composition comprising TiO2 and Li2TiO3 and a composition comprising TiO2, Li2TiO3 and Li4Ti5O12, and sintering the composition.
The method according to the invention will be explained in detail hereinafter.
The lithium titanate produced by the invention is represented by the general formula LiXTiYO12, in which the Li/Ti ratio is in the range of 0.78 to 0.82, X is in the range of 3 to 5, and Y is in the range of 4 to 6. In particular, the invention is directed to single phase lithium titanate with a spinel crystal structure presented by Li4Ti5O12, or a mixture or mixed crystal structure of Li4Ti5O12, Li2TiO3, and TiO2.
1. Raw Materials
The lithium compounds as raw materials for the invention are one or more lithium compounds selected from the group consisting of lithium carbonate, lithium hydroxide, lithium nitrate, and lithium oxide. Among these compounds, lithium carbonate and lithium hydroxide are preferably used. These lithium compounds used as materials preferably have high purity, normally of 99.0 weight % or more. For example, when lithium hydroxide is used as a raw material, Li2CO3 is preferably included at 99.0 weight % or more, and is more preferably 99.5 weight % or more; the content of Na, Ca, and Mg, etc., as impurities, is preferably 100 ppm or less, and is more preferably 10 ppm or less; and the content of Cl and SO4 is preferably 100 ppm or less, and is more preferably 50 ppm or less. The water component should be sufficiently removed, and the content thereof is preferably 0.1 weight % or less. Moreover, the average particle size is preferably in the range of 0.01 to 100 xcexcm, and in the case of lithium hydroxide, the particle size is preferably in the range of 1 to 50 xcexcm, and is more preferably in the range of 5 to 20 xcexcm.
With respect to titanium dioxide (TiO2), it should also have high purity. In particular, the purity is preferably 99.0 weight % or more, and is more preferably 99.5 weight % or more; the content of Fe, Al, Si, and Na included in fine particles of titanium dioxide as impurities is preferably less than 20 ppm respectively; and the content of Cl is preferably less than 200 ppm. More preferably, the content of Fe, Al, Si, and Na included in fine particles of titanium dioxide are less than 10 ppm, respectively; and the content of Cl is less than 100 ppm, and is more preferably less than 50 ppm. The average particle size is preferably in the range of 0.05 to 30 xcexcm, and is more preferably in the range of 0.1 to 10 xcexcm.
2. Preparation of Materials for Presintering
Lithium titanate and titanium dioxide as materials for presintering are sufficiently mixed, and are provided for presintering. The mixing ratio of the materials may be chosen to coincide with the value of the Li/Ti ratio of the lithium titanate in the desired final compound product. The term xe2x80x9cLi/Ti ratioxe2x80x9d refers to the atomic ratio unless otherwise specified. For example, when the Li/Ti ratio of the desired product is 0.78 or 0.80, the materials are mixed in a proportion of 0.78 or 0.80. In this case, a crushing mixer such as a vibrating mill or a ball mill, a mixer with an agitator, a rotating mixer, or the like is used. The moisture absorption during mixing should be monitored. The mixing is preferably performed in dry air with an absolute humidity of 5 g/m3 or less in air; alternatively, it may be performed in an inert gas. The mixture of the raw materials is provided to a presintering process in a powder state or as a green compact which is formed by compression at a pressure of 0.5 ton/cm2.
In this case, it is also a preferable aspect of the present invention that the lithium compound solution be prepared by dissolving all or part of the lithium compound in solvent and drying, and a mixture solid is therefore obtained and is then pre-sintered. The lithium compound solution is a homogeneous solution in which all of the lithium compound is dissolved in solvent, or a solution in which part of the lithium compound is dissolved in solvent and the other part is suspended in the solvent. As a solvent, optional solvents can be employed if the lithium compound can be dissolved, and water, alcohols, or mixtures of these, can be mentioned and in particular, water is preferred. Ratios of lithium compound to solvent are different depending on solubility of the lithium compound used to the solvent, and when water is used as a solvent, the amount of the solvent per 1 g of lithium compound is usually 1 ml or more, and is preferably 5 ml or more. The temperature for preparing the above lithium compound solution is not limited and in particular, the temperature is preferably room temperature to 100xc2x0 C., and is more preferably 50 to 80xc2x0 C., from the viewpoint of the acceleration of the dissolution.
In this aspect, the lithium compound solution and titanium dioxide can be mixed by the above mixing method. The mixing method may be performed by mixing solvent, lithium compound, and titanium dioxide; by bringing into contact therewith, the lithium compound is dissolved into the solvent, and the lithium compound solution is thereby prepared and is mixed with the titanium dioxide at the same time. In the above mixing, solvents other than the solvent used in the lithium compound solution can be used, and for example, alcohols such as methanol, ethanol, 2-ethylhexanol, etc., saturated hydrocarbons such as hexane, heptane, cyclohexane, etc., aromatic hydrocarbons such as benzene, toluene, xylene, ethylbenzene, etc., halogenated hydrocarbons such as o-dichlorobenzene, dichloromethane, tetrachloromethane, dichloroethane, etc., acetone, ethers, acetonitrile, tetrahydrofuran, etc., can be employed.
As described above, the lithium compound solution is mixed with titanium dioxide and is dried, and the mixture solid is therefore prepared. At this time, by evaporating the solvent, adding alcohols in which lithium compound is not dissolved, or replacing such solvent, the dissolved lithium compound is crystallized. Then, total solids in the solvent are dried, and the mixture solid of lithium compound and titanium dioxide is therefore obtained. Thus, in the wet-type mixing method as described above, since all or part of the lithium compound is dissolved and mixed with titanium dioxide, the lithium compound and the titanium dioxide are uniformly and finely dispersed, and reactivity thereof in presintering and sintering is made superior. As the result, lithium titanate, which has superior characteristics in batteries, can be obtained.
3. Presintering Process
In the invention, first, the mixture of raw materials is presintered under the conditions indicated below. That is, the mixture of the raw materials is heated and sintered in an oxidizing atmosphere at a temperature in the range of 600 to 800xc2x0 C., preferably in the range of 670 to 800xc2x0 C., and more preferably in the range of 700 to 780xc2x0 C. The duration of the heating and presintering is adequate at 30 minutes to 4 hours. The rate at which the temperature is increased is adequate in the range of 0.5 to 10xc2x0 C./min. By this presintering process, a presintered product (hereinafter referred to as the xe2x80x9cintermediate productxe2x80x9d) is obtained.
The chemical composition of the intermediate product is two components substantially consisting of TiO2 and Li2TiO3, or three components substantially consisting of TiO2, Li2TiO3, and Li4Ti5O12. The chemical composition of the intermediate product may be specified by a chart obtained by an X-ray powder diffraction analysis. In the chart, Li2TiO3 is specified at 43.4xc2x0, Li4Ti5O12 is specified at 18xc2x0 and 43xc2x0, and TiO2 (rutile type) is specified at 27xc2x0, and the presence of these is determined. Also the presence of the raw material, for example, Li2CO3, is determined according to detection of a peak at 31.5xc2x0. Thus, in the invention, the presence of the intermediate products and the raw materials is determined by whether or not these compounds are detected by the X-ray powder diffraction analysis. Therefore, the invention does not exclude cases in which components other than the above components are not detected by the X-ray powder diffraction analysis. In cases in which the relative intensity of the peak is less than 1 when the peak exhibiting the maximum intensity among plural detected peaks is defined as 100, it is determined that the component is not detected by the X-ray powder diffraction analysis.
According to the basic concept of the invention, it is important that the lithium compounds in the raw material be entirely converted to intermediate products after the presintering process and that a part of TiO2 as a raw material remain.
As a heating atmosphere for presintering, an oxidizing atmosphere is preferable such as oxygen gas, oxygen-containing gas, etc., or an atmosphere in which the partial pressure of oxygen gas is preferably 0.1 Pa or less, is more preferably 0.5 Pa or less, and is most preferably 1 Pa or less. In the present invention, it is one of the preferable aspects that presintering be carried out in the latter atmosphere in which the partial pressure of oxygen gas is 0.1 Pa or less and the amount of oxygen gas is very small, as well as sintering as described below, and lithium titanate in which discharge capacity is high and charge-discharge cycle property is highly superior when it is used as a negative electrode in a lithium ion battery, can be thereby produced. The atmosphere in which the partial pressure of oxygen gas is 1 Pa or less refers to a vacuum, an inert gas atmosphere such as nitrogen or argon, or a reducing gas atmosphere such as hydrogen gas, etc.
4. Sintering Process
In the invention, the intermediate product produced by the presintering process is subjected to a sintering process. In the sintering process, the intermediate product is heated to a temperature in the range of 800 to 950xc2x0 C., and preferably to a temperature in the range of 820 to 950xc2x0 C., more preferably to a temperature in the range of 850 to 930xc2x0 C. without removing the intermediate product from the furnace. Alternatively, the compact is removed from the furnace, and is crushed and mixed to again form a compact, and is then heated to a temperature for the sintering process. When the temperature for the sintering process is less than 800xc2x0 C., TiO2 does not sufficiently react, and tends to remain in the final desired product, lithium titanate such as Li4Ti5O12. When the temperature for the sintering process exceeds 950xc2x0 C., the chemical composition of Li4Ti5O12 is converted into other compounds, and therefore it is difficult to obtain the desired Li/Ti ratio of 0.78 to 0.82. The temperature for the sintering process is set to be a temperature higher than that of the presintering process.
The sintering process can be performed in an oxidizing atmosphere, and the sintering duration may be set in the range of 30 minutes to 10 hours. The temperature and the duration for the sintering process is preferably chosen based on the subsequent residual amount of TiO2. That is, the sintering process is performed so that the proportion of the intensity at the X-ray powder diffraction peak (27xc2x0) of TiO2 in the product after the sintering process with respect to the intensity at the X-ray powder diffraction peak (18xc2x0) of Li4Ti5O12, which is a relative intensity ratio (referred to as the xe2x80x9cTiO2 residual degreexe2x80x9d) and is 0.1 or less, preferably 0.05 or less, and more preferably 0.02 or less. The lithium titanate with such properties exhibit desirable battery properties when used in, for example, secondary lithium batteries.
In a preferable aspect according to the present invention, an intermediate product is sintered in an atmosphere in which the partial pressure of oxygen gas is 1 Pa or less, is preferably 0.5 Pa or less, and is more preferably 0.5 Pa or less. The atmosphere in which the partial pressure of oxygen gas is 1 Pa or less refers to a vacuum, an inert gas atmosphere such as nitrogen or argon, or a reducing gas atmosphere such as hydrogen gas, etc., and the atmosphere is preferably an inert gas atmosphere in which the partial pressure of oxygen gas is 1 Pa or less and is more preferably a nitrogen gas atmosphere in which the partial pressure of oxygen gas is 1 Pa or less. Thus, by sintering in an atmosphere in which the amount of oxygen gas is very small, lithium titanate in which discharge capacity is high and charge-discharge cycle property is highly superior can be produced, when it is ultimately used as a negative electrode or a positive electrode of a lithium ion battery.
The thus-obtained lithium titanate is removed from the sintering furnace and is subjected to suitable processes such as crushing, classification, and screening in a dry atmosphere, and the desired lithium titanate compound is thereby produced. The average particle size of lithium titanate is in the range of 0.1 to 15 xcexcm, preferably in the range of 0.3 to 10 xcexcm, and is more preferably in the range of 0.5 to 5 xcexcm.
As explained above, the invention is characterized in that the raw material, lithium compounds, and titanium dioxide, are selectively converted to the specific compositions in the presintering process, and lithium titanate having a Li/Ti ratio (atomic ratio) in the range of 0.78 to 0.82 is produced in the subsequent sintering process. The conventional processes require larger amounts of raw material in consideration of vaporization losses of the lithium components in the sintering process, so that the Li/Ti ratio is not easily controlled and is not uniform. In contrast, the invention can solve these problems.
Preferred embodiments of the invention are described below.
(1) The combination of the temperatures for the presintering process in the range of 650 to 800xc2x0 C. and for the sintering process in the range of 800 to 950xc2x0 C. is employed.
(2) The combination of the temperatures for the presintering process in the range of 670 to 780xc2x0 C. and for the sintering process in the range of 820 to 900xc2x0 C. is employed.
(3) The presintering process produces two components substantially consisting of Li2TiO3 and TiO2, or three components substantially consisting of Li2TiO3, Li4Ti5O12, and TiO2.
(4) The TiO2 residual degree is 0.1 or less.
The invention also provides a negative electrode produced from the above lithium titanate for a lithium ion battery. The electrode for the lithium titanate ion battery is produced by optionally adding electrode combination agents such as conductive agents and binders to the lithium titanate of the invention. In particular, it is possible to use conductive materials such as graphite, carbon black, acetylene black, Ketjen Black, carbon fiber, metallic powders such as those of copper, nickel, aluminum, silver, metal fiber, or polyphenylene derivatives. It is possible to use polysaccharides, thermoplastic resins, and elastic polymers, etc., as binders. Specifically, starch, polyvinyl alcohol, reconstituted cellulose, polyvinyl chlorides, polyfluorinated vinylidene, polyethylene, polypropylene, ethylene-propylene rubber, etc., can be used. A filler such as polypropylene or polyethylene can be added in addition to the above.
In addition, the invention provides a lithium ion battery using a negative electrode comprising the above lithium titanate. The lithium ion battery is composed of the negative electrode, a positive electrode, and electrolyte. Materials for the positive electrode are not limited, and well known materials can be used therein. For example, it is possible to use lithium manganate, lithium cobaltate, lithium nickelate, nickel inclusion lithium cobaltate, vanadium pentoxides, etc. The electrolyte used therefor is composed of a lithium salt and a solvent. Solvents such as propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, xcex3-butyrolactone, methyl formate, methyl acetate, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, dimethylsulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, and ethylmonoglyme can be used as solvents. It is possible to use LiPF6, LiClO4, LiCF3SO3, LiN(CF3SO2)2, and LiBF4, etc. as the lithium salt. The lithium salt is dissolved in the solvent to form an electrolyte, and the lithium ion battery of the invention is constructed by combining the positive electrode and the negative electrode therein.
As mentioned above, the invention can efficiently provide lithium titanate having the desired composition by the specified production conditions, and therefore can provide a negative electrode and lithium ion battery exhibiting high discharge capacity and superior charging and discharging properties by using the lithium titanate in the negative electrode in a lithium ion battery.
The above are preferred embodiments. Other embodiments will be clear from further a reading of the specification.